Sending an email from Roundcube results in: SMTP Error (-1): Connection to server failed.

Symptoms

Email cannot be sent from Roundcube and a message similar to the following is shown when attempting to send an email:

SMTP Error (-1): Connection to server failed in /usr/local/cpanel/base/3rdparty/roundcube/program/lib/Roundcube/rcube.php

An error similar to the following may also appear in the mail Exim log:

2022-11-28 22:55:43 TLS error on connection from [::1]:34936 (SSL_accept): timed out

Description

The tls_on_connect_ports Exim setting specifies a list of incoming SSMTP ports that should operate the obsolete SMTPS protocol and immediately set up a TLS session without waiting for the client to issue a STARTTLS command. When port 587 is added to the tls_on_connect_ports list, Roundcube is unable to connect to Exim.

Workaround

1. Log into WHM as the 'root' user.
2. Navigate to "Home / Service Configuration / Exim Configuration Manager."
3. Click the "Advanced Editor" tab.
4. Scroll down to the tls_on_connect_ports setting.
5. Remove 587 from the ports list, ensuring 465 is still in place
6. Scroll to the bottom of the page.
7. Click the "Save" button.

Additional resources

Exim Configuration Manager: Advanced Editor